Liquefied petroleum gas (LPG), primarily comprised of propane, may be used as a fuel for an internal combustion engine. LPG has a relatively low super critical temperature of about 96° C. If LPG is elevated to temperatures greater than its critical temperature, it may be supplied to an engine in an unknown density, between gaseous and liquid states. If LPG is supplied to the engine at temperatures less than its critical temperature, it may be supplied to engine fuel injectors in a liquid state. LPG exiting the fuel injector may exit the fuel injector and flash to a gaseous state with speed. Supplying LPG in a liquid state may be desirable because liquid fuel may be supplied directly into a cylinder where it evaporates and cools the cylinder air-fuel mixture so that the engine may tolerate additional spark advance and be less prone to engine knock. However, engine compartment temperatures may reach levels higher than the critical temperature of LPG. Consequently, there may be conditions when LPG changes state to supercritical before it is injected to the engine. The fuel's state change from liquid to supercritical may result in engine air-fuel ratio errors and an increase in the engine's propensity to knock when the gaseous fuel is injected to the engine.
The inventor herein has recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: supplying liquefied petroleum gas (LPG) in a liquid phase directly into a cylinder of an engine in response to a temperature of a fuel system being less than a threshold level; and ceasing to supply LPG directly into the cylinder and supplying LPG into an intake port of the cylinder in response to the temperature of the fuel system being greater than the threshold level.
By injecting fuel vapor from a fuel tank, it may be possible to provide the technical result of cooling fuel supplied to a direct injection fuel pump and fuel rail so that there may be a reduced possibility of fuel transitioning to a super-critical or gaseous phase. For example, if fuel pumped from a direct fuel injection pump begins to approach a temperature where the fuel changes state from liquid to gas, a portion of fuel in a direct injection fuel rail may be returned to a fuel storage tank to remove heat from the direct injection fuel rail. If a pressure in the direct injection fuel rail at a time before fuel is returned to the fuel tank is maintained at a time when fuel is being returned to the fuel tank, consistent fuel injection may be maintained while heat is removed from the direct injection fuel rail. The heat removed from the fuel rail and returned to the fuel tank may raise fuel tank temperature, thereby producing fuel vapors. Heat in the fuel tank may be reduced via injecting the fuel vapors from the fuel tank to the engine. In this way, heat may be removed from a fuel system so that fuel may stay in a liquid state at direct fuel injectors.
The present description may provide several advantages. In particular, the approach may reduce engine air-fuel ratio errors by allowing fuel to be injected in a known state. Further, the approach may remove heat from a direct injection fuel system so as to allow injection of fuel in a liquid state. Further still, the approach may also improve the way boost is provided to an engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.